Design, Operation and Optimization of Batch Reactive Distillation with Off-Cut

Batch reactive distillation (BREAD) is an integrated process which combines the advantages of reactive distillation and the flexibility of batch processes. Off-cut is commonly applied in non-reactive batch distillation to improve the performance. However, reports of the application of off-cut for BREAD are quite rare. In this thesis, several aspects of design and operation of BREAD processes with off-cut are studied. A hypothetical reaction system considered previously (Kao, Y. L. Effect of Relative Volatility Ranking on the Design and Operation of Batch Reactive Distillation System. Master Thesis. National Taiwan University, Taipei, Taiwan, 2010) is used to develop the optimal operation policy without off-cut. In addition, the effect of changing product purity specifications and reaction equilibrium constants is also investigated. Compared with the previous results, the new results show that using off-cut can improve performance in terms of batch capacity (CAP), especially when both reactants have boiling points between those of the two products, when the product purity is high, and when the reaction equilibrium constant is low. A real case study, hydrolysis of methyl lactate, is also investigated to validate the results from the ideal system study. Next, methods for improving the operation of BREAD with off-cut are presented. Although off-cut can improve the operation of batch reactive distillation, its usefulness is limited in some cases due to the relative volatility ranking of reactants and products. In these cases, performance can be improved by using a middle vessel column or by using an excess of one reactant. These methods are demonstrated with case studies of three processes with realistic reaction kinetics and vapor-liquid equilibrium models: hydrolysis of methyl lactate, esterification of formic acid, and production of 1,1-dimethoxyethane. CAP can be improved on average 41.9 percent if the appropriate method is employed. Off-cut recycling in BREAD processes is then studied. The strategy is to recycle off-cut to the next batch as a part of the initial feed. This transforms the operation problem from single batch problem into a batch-to-batch problem. The pseudo-steady-state concept is applied to BREAD processes to simplify the optimization problem. Optimization based on maximizing CAP for the batch-to-batch problem is demonstrated using three real chemistry processes. The results show that the optimal operating recipe and CAP when off-cut is recycled are similar to the case when off-cut is collected but not recycled, and the CAP is on average almost twice (93.3% more) the maximum that can be achieved when off-cut is not used. Therefore, recycling off-cut may not only save the trouble of processing the off-cut, but also make the process more economical. Last, simultaneous optimization of equipment design and process operation is investigated for BREAD processes. The minimal total annual cost problem is illustrated with two realistic chemical systems, hydrolysis of methyl lactate and esterification of formic acid. The effect of design and operating variables on TAC is investigated. The optimization results suggest that a reflux policy which maintains constant distillate composition can provide nearly optimal operation. The column should be designed with an adequate number of stages so that the CAP improvement by further increasing the number of stages is insignificant. Vapor boilup rate should be specified to make the best use of the available operating time, and the optimal vapor boilup rate is primarily affected by the catalyst loading. The insight into the effect of process design variables on design performance is used to develop an efficient algorithm for determining simultaneously the optimal column design and operating policy.